MINNEAPOLIS (WCCO) — The most striking feature on the iconic U.S. Bank Stadium in downtown Minneapolis is something you look right through — literally.

Instead of a 1990s-style retractable roof, which most NFL stadiums rarely open, the Minnesota Vikings are using a revolutionary design that is incredibly light.

“It’s truly going to feel like you’re at, you know, an open-air stadium, but you’ll be here in the winter months,” Mortenson Construction’s Brendan Moore said.

The colossal stadium is 1.8-million square feet. The entire old Metrodome could fit inside it.

And 60 percent of the slanted roof is covered with a product called ethylene tetrafluoroethylene, better known by the nickname ETFE.

Casey Ramsey, of German firm Vector Foiltec, led more than a dozen workers in installing the special roof panels 30 stories above the ground.

There are 75 separate sections, manufactured by Vector Foiltec, with each one measuring 10-by-300 feet, and weighing 1,200 pounds.

“Approximately 14 people, all with fall protection and safety rope training, they work together to pull the piece in one piece all the way down and snap it into place,” Ramsey said.

ETFE was developed in the 1970s by DuPont for insulation on wires and cables. It is 1/100-of-an-inch thick and a fraction of the weight of glass. Three gently-inflated plastic layers give it flexibility and durability.

And like its cousin, Teflon, ETFE has chemical properties allowing rain and snow to slide right off.

“And there’s nothing like it, literally, in the world,” Michele Kelm-Helgen, chair of the Minnesota Sports Facility Authority, said.

She says using ETFE reduced the steel load on the stadium roof by 1,000 tons. That is equal to six Boeing 767 aircraft.

Unlike the Metrodome, this roof will not collapse from the weight of snow and ice.

“That steep roof will allow it to shed the snow, and it will come down to these snow gutters, and there is a melt system within these snow gutters that we will use,” Kelm-Helgen said.

ETFE is already popular in stadium design around the world. The Olympics Water Cube in Beijing is made of it. And ETFE gave Munich’s Allianz Arena its shape-shifting appearance.

Minnesota’s ETFE roof will not be lighted, but the spaces around it will, giving it a glow of changing colors — including Vikings purple.

The 248,000-square feet of roof sits atop 10-story glass doors facing downtown Minneapolis, which will open and close.

ETFE provides a warming thermal effect in harsh weather, in what stadium builders say is an overall “feel.”

“Having an ETFE roof will allow the fans to benefit from the comfort of an indoor environment, while still having an open-air feel year round,” Ramsey said.

Now that the outside roof and walls are mostly complete, workers will concentrate on the inside of the stadium during the winter. It should open next summer in time for the NFL season.

The cost of a retractable stadium roof made it impractical. ETFE is lighter and less expensive.

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